1. Field of the Invention
The present invention relates to a solid-state image pickup device used in an image reading unit of a copying machine, facsimile apparatus, or the like.
2. Related Background Art
As conventional solid-state image pickup devices, various elements such as CCD type, photodiode type, bipolar transistor type, MOS type, and the like have been developed and used as solid-state image pickup elements and photoelectric conversion elements. Also, these elements are used as a line sensor obtained by arranging a large number of these elements in one line on a semiconductor chip and as an area sensor obtained by arranging these elements two-dimensionally. The line sensor is used in an image sensor, copying machine, facsimile apparatus, and the like. More specifically, the line sensor reads an original image by moving an original or a sensor system and transfers an image signal to a photosensitive body to copy it, or reads an original image in units of lines and writes the original image signal on a recording paper sheet. As for the area sensor, various applications such as a video camera, magnifying camera, video microscope, and the like are expected promising for the multimedia era in future.
For example, such solid-state image pickup device is constituted by the solid-state image pickup element for photoelectrically converting an image to generate pixel charges, a storage means such as a capacitor for temporarily storing pixel charges for one line in units of pixels, and a transfer means for time-serially outputting the pixel charges in turn in accordance with timing signals from a horizontal scanning circuit.
In general, in such solid-state image pickup device such as a line sensor, area sensor, or the like that uses a plurality of photoelectric conversion elements as pixels, some of a large number of pixels are shielded from light as optical black pixels (to be referred to as "OB pixels" hereinafter), an OB pixel signal is used as a reference output, and the difference between the reference output and the other non-shielded pixel signal is calculated and used as a regular sensor signal. As the method of using the signal from the OB pixel as a reference output, for example, a clamp circuit shown in FIG. 2A is known. In FIG. 2A, the clamp circuit includes a clamp capacitor 21 and a switch transistor 22.
When a sensor output shown in FIG. 2B is input to this clamp circuit, and the pulse .phi.CLP is applied during the OB output period, as shown in FIG. 2B, the sensor output of the OB pixel becomes a reference signal, and components indicated by hatching in FIG. 2B as the differences between regular sensor outputs S1, S2, S3, and the like, and the reference signal are output as effective signal output components.
When a plurality of OB pixels are arranged, the signals from these OB pixels are directly output in turn in units of pixels.
For this reason, when the clamp circuit is used, the reference output of the OB pixel is determined based on the output when the clamp pulse .phi.CLP changes from High to Low. Hence, even when the application time of the clamp pulse .phi.CLP is changed in correspondence with three OB pixels shown in FIG. 2C, the output from only one pixel (in this case, an OB pixel OB3) is used as the reference output.
The output from the shielded OB pixel includes random noise. Therefore, when the output from a single OB pixel is used as a reference output in this manner, the random noise produced by a specific OB pixel is reflected in the reference output. The random noise in the reference output corresponds to, e.g., random changes in reference output every time data for one line is read from the line sensor, and the output image suffers density nonuniformity at every line in the scanning direction of the sensor.